The field of the disclosure relates generally to methods and systems for use in controlling operation of a wind turbine, and more specifically, to controlling the operation of a wind turbine using dynamic braking in response to an islanding event.
Generally, wind turbine systems regulate a positive sequence voltage with a closed-loop current regulation scheme that minimizes negative sequence current. Such systems work well and are known to be reliable at constant output power levels. However, as the length of transmission line feeder to the DFIG wind turbine system is increased, response to grid transients and grid disturbances causes oscillations of power into and out of the converter which create disturbances on the DC bus voltage in the converter. Such power oscillations may, over time, lead to degradation of system controllability and/or equipment malfunctions. In some grid faults, upon clearing, the wind plant is left with no remaining connection to the grid, but still with the wind turbines connected to the cables and lines of the wind plant and at least a portion of a long transmission grid. This can be considered an “islanded” condition for the wind park that is characterized by potentially significant deviations in voltage and frequency. This condition is not to be confused with other usages of the term “islanding,” where the intent is to ensure safety of maintenance personnel.
The above-described events pose a potential for damage to the wind turbine electrical system due to high voltages within that system that exceed equipment capability. It is desirable for the wind turbine to ride through the grid events, both low-voltage and high-voltage, when the grid remains partially intact after clearing the grid fault. However, when the grid becomes open-circuited after clearing the fault, then it is desirable that the wind turbines continue operating without damage and eventually shut down based on inability to transfer power.
One control method for regulating the power flow during some grid disturbances involves operating a “rotor crowbar,” which is used as a last resort to limit power flow into the DC bus of the converter to keep the converter from being damaged. Generally; however, such a system does not allow the wind turbine system to recover fast enough to meet the some grid code standards and/or regulations. With existing control methods, as longer transmission line lengths are desired, possibly coupled with larger grid voltage transients, the voltage overshoots on the DC bus voltage in the converter may reach a level to damage the components in the converter.
Accordingly, a need exists to more effectively control wind turbine systems using dynamic braking to protect electrical equipment from disturbances caused by the power grid.